Conventionally, stroboscopic devices equipped with a flash discharge tube have been frequently used as a light source for emitting light to a photographic subject in taking photographs. Some of the stroboscopic devices are capable of selecting a flat light emission mode that causes the flash discharge tube to continue light emission for a predetermined period at an approximately constant luminance, or a normal light emission mode. In the above stroboscopic device, when, for example, the flat light emission mode is selected, a light emission control switching element such as an insulated gate bipolar transistor (hereinafter, abbreviated as IGBT) connected to the flash discharge tube in series is on-off controlled. This makes the flash discharge tube emit light at an approximately constant luminance in a pseudo manner for a predetermined period.
Hereinafter, operation of the stroboscopic device having the above configuration will be described with reference to FIGS. 6A and 6B.
FIG. 6A is a diagram illustrating a waveform of a light emission control signal input from an external signal input terminal of the stroboscopic device. FIG. 6B is a diagram illustrating a waveform of a driving voltage applied to the gate terminal of an IGBT in a conventional stroboscopic device.
Light emission control signal S as illustrated in FIG. 6A is typically input to a light emission control switching element of the stroboscopic device from the external signal input terminal of the stroboscopic device. This makes the light emission control switching element of the stroboscopic device perform ON/OFF operation to make the flash discharge tube perform light emission. Note that light emission control signal S is a control signal for applying a voltage that switches the light emission control switching element from an OFF state to an ON state. For example, when light emission control signal S is ON during time t1, the light emission control switching element holds the ON state during the period. Specifically, light emission control signal S is input from a control circuit for controlling the light emission amount of the flash discharge tube to the external signal input terminal.
Upon input of light emission control signal S from the control circuit, a driving voltage that increases at a predetermined voltage increase rate per a unit time is applied to the light emission control switching element as illustrated in FIG. 6B. Then, when the driving voltage applied to the light emission control switching element exceeds a predetermined threshold value voltage (for example, VGE), the light emission control switching element switches from an OFF state to an ON state, making a current flow in the flash discharge tube to start light emission.
At this time, a current flows in the flash discharge tube from a main capacitor via a current limiting coil. The current limiting coil is provided to slow the increase and decrease of the current flowing in the flash discharge tube (to lower change in light amount of light emission amount per a unit time).
That is, the current limiting coil slows the increase of the current flowing in the flash discharge tube when the light emission control switching element switches from the OFF state to the ON state. The light emission amount of the flash discharge tube also gently increases in response to the increase of the current.
The current limiting coil also slows the decreases of the current flowing in the flash discharge tube when the light emission control switching element switches from the ON state to the OFF state. The light emission amount of the flash discharge tube also slowly decreases in response to the decrease of the current. At this time, the light emission control switching element is switched to the OFF state, so that the current that has flowed in the flash discharge tube fails to flow via the light emission control switching element. Consequently, the current that has flowed in the flash discharge tube returns to the current limiting coil via a reflux diode connected in parallel with the flash discharge tube and the current limiting coil in the reverse direction.
At this time, noise generates in some cases when the light emission control switching element switches. Then, when the driving voltage applied to the light emission control switching element rises or drops more than or less than threshold value voltage VGE due to the generated noise, the ON/OFF state of the light emission control switching element is unintentionally switched in some cases. Furthermore, as illustrated in FIG. 6B, ON operation and OFF operation of the light emission control switching element is repeated due to noise in some cases. This unfortunately results in, for example, breakage of the light emission control switching element.
In order to solve the above problem, a stroboscopic device equipped with the configuration as described below is disclosed (for example, see PTL 1).
The stroboscopic device described in PTL 1 includes a light emission tube, an IGBT, a gate voltage generating circuit, and a gate voltage degenerating circuit, a timer, and an AND gate. The light emission tube corresponds to the above “flash discharge tube”. The IGBT corresponds to the above “light emission control switching element” and is connected to the light emission tube in series to control light emission. The gate voltage generating circuit applies a voltage to the gage of the IGBT in response to a light emission start signal corresponding to above “light emission control signal S in ON state”. The gate voltage degenerating circuit degenerates the gate voltage of the IGBT in response to a light emission stop signal corresponding to above “light emission control signal S in OFF state”. The timer and the AND gate cancels the light emission stop signal during a predetermined period in response to the output of the light emission start signal. At this time, the timer outputs an output signal of “low (hereinafter, abbreviated as L)” level for a constant period in response to an input signal. Then, the AND gate outputs a result of the logical multiplication of the level of the output signal from the timer and the level of the light emission stop signal.
That is, in the above stroboscopic device, first, the gate voltage generating circuit applies a voltage to the gate of the IGBT in response to the input light emission start signal. At the same time, the light emission start signal is also input to the timer.
In this case, even when the light emission stop signal is input while the output signal from the timer is in “L” level, the output signal from the timer is input to the AND gate at “L” level. Consequently, the output signal from the AND gate is kept to “L” level regardless of input of the light emission stop signal. This allows the above stroboscopic device to cancel the light emission stop signal to be input till the output signal from the timer is switched to “high (hereinafter, abbreviated as H)” level. This makes it possible to keep light emission of the flash discharge tube while the output signal from the timer is in “L” level.
A stroboscopic device is also disclosed that includes a flash discharge tube, a flash discharge tube driving circuit using an IGBT, a timer circuit, and a flash discharge tube lighting control circuit (for example, see PTL 2). Note that, the timer circuit has a function for holding the conduction of the IGBT during a set period. The flash discharge tube lighting control circuit controls the conduction to the flash discharge tube by the logical sum of the output from the flash discharge tube driving circuit and the output from the timer circuit.
That is, the above stroboscopic device first divides the input signal corresponding to the above “light emission control signal S” to be input from an external signal input terminal into two signals, and input one of the divided input signals to the flash discharge tube driving circuit and input the other one of the divided input signals to the timer circuit.
Then, the timer circuit outputs a pulse (rectangular wave) having the length same as a pulse output from the flash discharge tube driving circuit in response to the input input signal.
Then, the output signals from the flash discharge tube driving circuit and the timer circuit are input to an OR (logical sum) gate. Then, a result of the logical sum is input to the IGBT.
Note that the flash discharge tube driving circuit of the above stroboscopic device is readily affected by noise as compared with the timer circuit. This is because, as illustrated in FIG. 6B, the rise time of the driving signal is slow in the flash discharge tube driving circuit, so that the noise generated at switching of the IGBT is readily superimposed, whereas the output from the timer circuit has a short rise time, so that noise is not easily superimposed. In view of such characteristics, the output signal of the timer circuit is switched to “H” level. This makes it possible to apply voltage to the gate of the IGBT regardless of the output signal from the flash discharge tube driving circuit affected by noise, allowing the flash discharge tube to keep light emission while the timer circuit is in “H” level.
However, the stroboscopic device described in PTL 1 needs an extra circuit configuration such as a timer and an AND gate to cancel the light emission stop signal affected by noise. Likewise, the stroboscopic device described in PTL 2 unfortunately needs an extra circuit configuration such as a timer circuit and an OR (logical sum) gate to cancel the output signal from the flash discharge tube driving circuit affected by noise.